Hydrology in the Humid Tropic Environment (Proceedings of a symposium held at Kingston, Jamaica, November 1996). IAHS Publ. no. 253, 1998. 435 Dissolved matter fluxes in the inner delta of the River Niger J. P. BRICQUET, L. GOURCY, G. MAHÉ, D. ORANGE, C. PICOUET & J.-C. OLIVRY Laboratoire Earn Continentales, Centre ORSTOM, BP 84, Bamako, Mali Abstract This study is part of the EQUANIS programme, the objectives of which are (a) estimating inputs from the River Niger to the inner Delta; (b) computing the hydroiogical balance; and (c) monitoring the quality of water in the central lacustrine basin. Eleven sampling locations were selected near gauging stations, both on the Niger River and its main tributary, the Bani River. Those sites have been sampled weekly since July 1990. The inner delta of the Niger River is a system particularly subject to sahelian and sub-desert climatic conditions and characterized by large flood plains. Time series of input water volumes in the inner delta, and of the water losses within it, show that the water losses are high, due to the intense evaporation, and vary from 6 to 40 km3. The water losses reach their maximum during the wettest years—up to 47%, and minimum during the driest years—only 32%, due to the reduction of the flooded area. The surface of the flooded area is inferred from the hydroiogical balance. The preliminary results of this study indicate that the Niger and Bani rivers have low levels of dissolved element concentrations. The mean conductivity values, ranging from 50-80 uS cm"1, increase regularly during the low water stage and decrease drastically with rising water. The pH values are slightly basic, ranging from 7.1 to 8. Silica and bicarbonates are the main dissolved species; they always represent more than 75% of TDS. In May, when the rising water stage begins, the waters are poorly mineralized at the input of the delta, while they have been enriched during the dry season in the delta. A good mass balance is found between inputs and outputs through the delta. However, a disequilibrium appears at the sampling sites within the basin, which could be partially linked to poor mixing between the Niger and Bani river waters. The first results from the upper basin and below the inner delta, show low concentrations of matter. The specific dissolved loads vary between 10-12 t km"2 year"1 for the Niger River and 2.5 t km"2 year"1 for the Bani River. The annual input in the inner delta was about 2.2 x 1061 in 1992- 1993. Chemical budgets show a saline deposit of 0.3 x 1061 in the inner delta. Seasonal variations of the dissolved matter fluxes are very different between the upper and lower parts of the inner delta, due to the breaking of the annual flood and to the more important flood plains in the upper delta. Los flujos de materiales disueltos en el delta inerior del Niger Resumen Es présente estudio forma parte del programa EQUANIS, cuyos objetivos son (a) estimation de los aportes del rio Niger al delta interior, (b) cômputo del balance gîdrico, y (c) monitoreo de la calidad del agua en la cuenca lacustre central. Se seleccionaron once sitios de muestreo prôximos a las estaciones de aforo tanto en el Niger como en su principal afluente, el Bani. Desde julio de 1990 se llevaron a cabo alii muestreos semanales. El delta interior del Niger is us sistema particular sometido a condiciones climâticas "sahelianas" y subdesérticas, y se caracteriza por grandes planicies de inundation. Las series temporales de aporte hidrico in il delta 436 J. P. Bricquet et al. interior, y de pérdidas de agua en is mismo, muestran que las pérdidas son elevadas debido a la evaporaciôn intensa, y fluctûan entre 40 y 6 km3. Las pérdidas de agua son mâximas durante los anos mâs hûmedos (hasta un 47%) y mïnimas durante los anos mâs socos (solo 32%) debido a la reducciôn de las areas inundadas. La superficie inundada se deduce del balance hïdrico. Los resultados preliminares del estudio indican que los rîos Niger y Bani tienen un bajo nivel de concentraciôn de elementos en disoluciôn. Los valores medios de conductividad que van de 50 a 80 u.S cm'1 se incrementan regularmente durante el perîodo de estiaje y disminuyen drâsticamente en periodos de avenidas y variacîon de nivelés. Los valores de pH son ligeramente bâsicos, de 7.1 a 8. Los principales materiales dusueltos son el silice y los bicarbonatos; siempre representan mâs del 75% del TDS. En mayo, cuando se inicia el perîodo de crecida, las aguas contienen pocos minérales en la entrada del delta mientras que se han concentrado durante la estaciôn seca del delta. Se observa un buen balance de masa en el delta entre los aportes y descargas, pero aparece un desequilibrio en los sitios de muestreo dentro de la cuenca, que puede atribuirse parcialmente a una mala mezcla entre las aguals del Niger y del Bani. Los primeros resultados en la cuenca superior y baja del delta interior muestran bajas concentraciones de material. Las cargas disueltas especïficas fluctûan entre 10 y 12 t km"2 ano"' para el rîo Niger y 2.5 t km"2 ano"1 para el rîo Bani. El aporte anual en el delta interior fue de unos 2.2 x 1061 en 1992-1993. Los balances quîmicos presentan un depôsito salino de 0.3 x 1061 en el delta interior. Las variaciones estacionales de los flujos de materiales son muy diferentes entre las partes alta y baja del delta interior, debido al inicio de la crecida anual y a las mâs importantes planicies de inundation en la parte superior del delta. INTRODUCTION Since 1990, the EQUANIS project on the environment and quality of contributions of the Niger River to the Sahel has been developed in Mali to establish a network of observation stations of flows of suspended and dissolved matter. The project has been gradually developing the theme of research of the PEGI programme (INSU, Institut National Scientifique de l'Univers; CNRS, Centre National de Recherche Scientifique; ORSTOM). This programme combines several subjects: - the estimation of the solid and liquid contributions of the Niger River to the central delta; - modelling of the water balance of the inner delta of the Niger using calculations of water losses and the evolution of concentration of dissolved matter to establish a relationship between flooded surface and duration of flood; - the supervision of the environment and the water quality to detect major alterations of the system; - the establishment of relationships between renewable hydrological resources and renewable living resources. The network depends on the knowledge of water transfers well identified by reference gauging stations (Brunet-Moret et al., 1986). Measurement protocols have been adapted to particular conditions of the hydrographie system of the Niger River, notably in the lacustrine basin where flows are particularly slow and where the vast flood plains allow the observation of deposits and the evolution of dissolved matter flows. Dissolved matter fluxes in the inner delta of the River Niger 437 The regime of export of dissolved matter of the basins is being studied at the Ke- Macina station (see Fig. l)(after withdrawal of canal flows by the Office of Niger for Fig. 1 EQUANIS project area, with maximum limit of the flooded area and permanent rivers. 438 /. P. Bricquet et al. irrigation, and by the Markala Dam), and at the Douna station on the Bani River, principal tributary of the Niger (basin of 102 000 km2 in natural regime). These two stations have been retained as references of input flows in the lacustrine basin, i.e. the inner Delta of the Niger River. The study of the main outflows of the Niger-Bani system in the inner Delta is assigned to Kara station on the Diaka River, Tilembeya and Mopti stations on the Niger River, Sofara station on the Bani River, before the waters of the upstream basin meet in Lake Débo. A first budget for outputs from Lake Débo is calculated using the three outflows of the lake from Aka station on the Issa Ber, Awoye station on the Bara Issa and Korientze station on the Koli-Koli. These three outflows form a unique reach level upstream to Dire; it is at this station that the total outputs of the inner delta are controlled. STUDY SITE AND METHODOLOGY The lacustrine basin of the Niger River The Niger, a great African river, crosses right through Mali and, in the centre of the country, floods an area of some thousands of square kilometres. The river has its source on the humid Guinean plains and, downstream of Mopti, receives the Bani River coming from the less watered regions of Ivory Coast. The long-term mean annual discharge values for the period 1935-1995 were 1200 m3 s"1 at Ké-Macina, 335 m3 s"1 at Douna (Bani), 992 m3 s"1 at Mopti (junction of the two rivers) and 971 m3 s"1 at the exit of the basin at Dire (Fig. 2). The hydrograph is tropical sudanian (Brunet-Moret et al., 1986). The hydraulic slope is 7 cm km"1 upstream of Ké-Macina and rapidly falls to 4 cm km"1. The slope downstream of Lake Débo does not exceed 0.7 cm km"1. This m3 s-1 2000 -, < i i i i i— 1924 1934 1944 1954 1964 1974 1984 Hydrological year Fig. 2 Annual discharge variation for uie Niger River basin. Dissolved matter fluxes in the inner delta of the River Niger 439 provokes a slowing that, during the annual flood, gives rise to a deposit of the load and overflow of the waters.